Unsaturated Polyester Resins

8/12/2019 Unsaturated Polyester Resins

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Making High Performance Unsaturated Polyester ResinsWith 2-Methyl-1,3-Propanediol

Lau Yang, Mac Puckett

Lyondell Chemical Company1221 McKinney Street

Houston, TX 77253

Abstract: Unsaturated polyester resins (UPR’s) have been made by reaction of 2-methyl-1,3-

propanediol (MPDiol) with all three of the common aromatic acids, ortho-, iso-, and terephthalicacid. The use of this unique glycol allows production of polymers with improved strength and

elongation and better caustic corrosion resistance than the resins more routinely made frompropylene glycol (PG). Additionally, formation of a UPR by reacting MPDiol and cheap, readilyavailable, recycled PET (polyethylene terephthalate) and maleic anhydride produces a polymer

with exceptional performance to price ratio.

Introduction: Unsaturated Polyester Resins (UPR’s) are the most commonly used thermosetresins in the world. There are approximately 2.2 billion kilos/year of UPR used around the globe

in the manufacture of a wide assortment of products including sinks, shower stalls, pipes, tanks,gratings, and high performance components for boats, buses, trucks, trailers and automobiles.These versatile resins are composed of a family of materials that are made from aromatic acids

(isophthalic, terephthalic, and phthalic anhydride), reacted with a glycol (typically propyleneglycol) or mixture of glycols, and maleic anhydride. The polymer thus generated is diluted withstyrene monomer to adjust the viscosity and form the final resin composition.

The Ortho resins (made from phthalic anhydride) are the most inexpensive class of resin and areused when the structural and corrosion requirements of the part are low. The Iso resins have

very good structural and corrosion properties but with their relatively high cost are used only indemanding applications. The Terephthalic resins are currently made in small volumes and are

considered a specialty resin. A major reason for the limited availability of Terephthalic resins isthe difficulty in making these resins from terephthalic acid (TPA) and propylene glycol (PG). Eventhough PG is the predominant glycol used in producing all types of UPR’s other glycols are usedincluding neopentyl glycol (NPG), diethylene glycol (DEG), and ethylene glycol (EG). Each of

these glycols when used in production of a polyester resin makes a contribution to the final set of polymer characteristics, including HDT, water uptake, strength, weatherability, etc.

Figure 1. MPDiol has a single pendantmethyl group along the glycol backbone.

A glycol that has become available recently is

MPDiol®

glycol (2-Methyl-1,3-propanediol). It

brings to polyester production a variety of advantages including both improvements in

processing and in final polymeric properties.MPDiol contains two very reactive primary

hydroxyls. (See Figure 1) It is thereforeinherently more reactive than the industrystandard glycol, propylene glycol (PG) whichhas one primary and one secondary hydroxyl.

The “side-chain” methyl group of MPDiol when spaced along a polyester polymer backboneinhibits formation of crystalline segments and a more styrene soluble material is realized. Evenwhen the exceptionally crystalline material terephthalic acid is used in the formulation, the unique

properties of MPDiol generate a clear styrene soluble UPR. The structure and reactivity of MPDiol also have a positive impact on the molecular weight and distance between crosslinkingpoints of the polyester polymers formed when the UPR is made. These improved polymers

HO OH

H CH3



routinely have greater tensile strength and elongation to failure values than similar UPR’s madewith PG or NPG. Table 1 summarizes the glycol's physical properties.

Table 1. Typical properties for MPDiol.

Physical State LiquidMelting Point - 60

oC

Boiling Point 213 o

C

Density (@ 20oC) 1.02 g/cc

Viscosity (@ 20 o

C) 178 mPa.s

Flash Point >110oC

Solubility in Water 100 %Vapor Pressure (@ 100

oC) 4.3mm Hg

Refractive Index (@ 20oC) 1.445

MPDiol can be used to make unsaturatedpolyester resins in one stage or two stagereactions similar to the way other glycols would

be used. Appropriate adjustments must bemade to initial charge ratios based onmolecular weight differences between glycols,

and adjustments in the processing conditionsmust be made to account for the high reactivityof MPDiol. The relatively high reactivity of

Phthalic Anhydride (1 stage reaction) andIsophthalic Acid (2 stage reaction) make theuse of a catalyst unnecessary. Because

Terephthalic Acid (2 stage reaction) has very

low solubility in most glycols and therefore tends to be much slower reacting even with MPDiol atelevated temperatures, the use of a catalyst such as an organo tin oxide like FASCAT 4100 or 9100 is recommended.

Results and Discussion: An overview of the synthesis and properties of the UPR’s made

with MPDiol are given for the three different classes of resin (Ortho, Iso, and Tere). Properties of commercially available Ortho and Iso resins are provided as a standard of comparison for these

resins. A typical reactor set up for running these types of reactions can be seen in Figure 2.

Synthesis and Properties of an Ortho Resin

The synthesis of Ortho resins from MPDiol isstraightforward, and is done via a typical onepot one stage reaction, which requires no

added catalyst. The high reactivity of MPDiol,makes it possible to drive the reaction with the

second acid group on phthalic anhydride (PA).The end result of the increased reactivity is thata higher molecular weight polyester polymer isformed than would normally be made by

reaction of PA and PG. The average molecular weight (Mn) observed in a series of runs withMPDiol was 2000-2400 with a polydispersity of

about 3.0. A typical Ortho resin made withPA/PG would have a Mn of about 2000 or lesswith a broader polydispersity (>4). Molecular

weight of a polymer is a major factor indetermining the thermal-mechanical propertiesof a resin. Therefore, a series of Ortho resins

made with MPDiol demonstrated exceptionalproperties. The caustic corrosion resistance of these finished resins is also dramatically

improved. (Notice the excellent causticcorrosion performance observed for the Isoand Tere resins made with MPDiol.) The

mechanical properties of a typical MPDiolmodified Ortho resin are shown in Table 2 andcompared to a commercial UPR.

Figure 2. Typical reactor set up.



Table 2. Comparative Properties of a Standard (Commercial) Ortho Resin and a Modified

Ortho Resin (made with MPDiol replacing PG).

Property MPD Modified Ortho Standard OrthoPA/ MA / MPD / PG 0.8/1.2/1.6/0.5 0.8/1.2/--/2.1

Tensile Strength (MPa) 76 55

Tensile Modulus (MPa) 3725 4000

Tensile Elongation (%) 3.5 1.2

Flexural Strength (MPa) 138 110

Flexural Modulus (MPa) 3725 4410

HDT (oC) 110 110

Water boil (% retention) 61 55

KOH boil (% retention) 61 25

HCl boil (% retention) 52 50

Viscosity at 45% styrene (cps) 400 200

Synthesis and Properties of an Iso ResinThe preparation of the Iso resin is a very straightforward one pot two stage reaction. With therelatively high reactivity of the Isophthalic acid and the MPDiol in the reaction, it is not necessary

to add a catalyst. The resin produced has low color and good cure reactivity. The thermal andmechanical properties of the polymer are as expected, see Table 2. Once again the averagemolecular weight (Mn) observed in a series of runs was about 2000-2500 with a polydispersity of

3.0. The greater length of the MPDiol molecule versus PG molecule produces a polyester polymer of greater ductility which expresses itself as an improvement in tensile elongation and areduction in HDT. The polymer corrosion resistance in water and acid are essentially unchanged

by the presence of MPDiol. However, the caustic corrosion resistance of the UPR is markedly

improved.

Table 3. Comparative Properties of a Standard (Commercial) Iso Resin and a Modified IsoResin (made with MPDiol replacing PG).

PropertyIPA/MA/MPD/PG

MPD Modified Iso

1.0/1.0/2.1/---

Std Iso/PG

1.0/1.0/---/2.1




Flexural Strength (MPa) 152 138Flexural Modulus (MPa) 3930 3800

HDT (oC) 91 106







Synthesis and Properties of a Terephthalate ResinWhen using MPDiol, preparation of the terephthalate resin is a remarkably easy one pot two

stage reaction. Terephthalic acid (TPA) is a very insoluble and non reactive material. Even whenusing a reactive glycol like MPDiol and taking advantage of its high boiling point, best results areobtained by using a small amount (approximately 100 ppm) of catalyst in the reaction. This

esterification can be completed with no catalyst present, but addition of an organo tin oxidecatalyst (e.g. FASCAT 4100 or 9100) is the most efficient method to complete the reaction. Thefinished resin is a low color material with high curing reactivity. The thermal and mechanical

properties of the polymer are excellent yielding a resin with properties that are comparable tothose of an Iso type resin, see Table 3. The corrosion resistance of this polymer in water, acid,and caustic are excellent. Table 4 provides properties for two Terephthalate polymers made with

different ratios of TPA and Maleic Anhydride.

Table 4. Comparative Properties of two Terephthalate Resins made with MPDiol replacing

most of the PG and having different ratios of TPA/MA.

Property Terephthalate* Terephthalate

TPA / MA / MPD / PG 0.8/1.2/1.6/0.5 1.0/1.0/1.6/0.5Tensile Strength (MPa) 64 89





HDT (oC) 120 99





* Material described in reaction procedure

Synthesis and Properties of a UPR made from post consumer recycled PET

The combinations of MPDiol, Terephthalate, and Maleic Anhydride that produce unsaturatedpolyester resins, make polymers that demonstrate excellent thermal, mechanical, and corrosion

properties. One of the very cheapest sources of the terephthalate moiety that can be used inmaking a UPR is post consumer scrap PET, available as recycled PET. The reaction of MPDiolwith PET is a straight forward transesterification reaction that breaks down (digests) the high

molecular weight PET polymer to make a very low molecular weight terephthalate diester of MPDiol and EG groups. The lowest molecular weight terephthalate product is made when an

equimolar ratio of MPDiol and EG is achieved. Reaction of this Stage 1 digestion product withmaleic anhydride followed by dilution with styrene forms an unsaturated polyester resin. Theproperties of a typical UPR of this type are shown in Table 5. The UPR resin labeled asTerephthalate A is a polymer that was made with an equimolar mixture of MPDiol and EG. The

thermal and mechanical properties of this resin looked very good and comparable to the resinsshown in Tables 3 and 4. The corrosion data however seems to show an unexpectedly large lossparticularly in caustic corrosion resistance.

The data in Table 5 labeled Terephthalate B was obtained from a resin made in a multistageprocess. The Terephthalate diester of MPDiol was first made by reaction of 2 equivalents of



MPDiol with 1 equivalent of dimethylterephthalate. This Terephthalate Diol was then used in

reaction with PET to form the “Stage 1-type” product. The advantages to this more complexprocedure are that the ratio of MPDiol to EG is altered in this way to a 3/1 ratio. And from aprocessing standpoint, the use of a higher molecular weight Terephthalate Diol versus MPDiol

directly, means a larger volume of liquid is used in reaction with the solid PET and the first stagereaction is easier. The properties of the finished product, Terephthalate B, are excellent acrossthe board. The thermal, mechanical, and corrosion properties of Terephthalate B compare very

well with those obtained from the Terephthalate and Iso resins found in Tables 3 and 4.Therefore, the combination of Terephthalate and MPDiol forms polymers with excellent thermaland mechanical properties over a fairly wide range, but in order to maximize the corrosion

properties of a Terephthalate resin, MPDiol must be the predominant glycol in the mixture.

Table 5. Comparative Properties of two Terephthalate Resins made from reaction of PET

and MPDiol having different ratios of MPDiol/EG.

Property Terephthalate A Terephthalate B

“TPA” / MA / MPD /EG/ PG 0.8/1.2/0.8/0.8/0.5 0.8/1.2/1.2/0.4/0.5






HDT (oC) 117 117





Procedure for UPR casting and Corrosion Testing

All the resins made and studied (Tables 2-5) were cured with 1% DDM-9 (MEKP) and 0.2%CoNap (6% solution in mineral spirits) overnight under ambient conditions, followed by a postcurefor 5 hours at 100

oC. The physical properties of the cured thermosetting polymers were

determined using ASTM test methods. Tensile strength, modulus, and elongation are determinedusing ASTM D-638, Type 1. Flexural strength and flexural modulus: ASTM D-790. DTUL: ASTM D-648. Short term environmental testing was performed by placing flexural test

specimens in a sealed tube with the indicated solvent for one week at 100oC. Following the high

temperature exposure the samples were removed and flexural tests were run to determine thepercentage of initial flexural strength the sample retained.

Comparison of Glycols in UPR production - Reactivity and Properties

A series of reactions were run to determine the relative reactivity of MPDiol versus PG with

terephthalic acid or isophthalic acid. The result of the comparison is shown in Figure 3. Thehigher boiling point of MPDiol and the greater inherent reactivity of the system result in reactions

run with MPDiol being about 4 times as fast as those run with PG. A comparison was also madeto evaluate the reaction of different glycols reacting with terephthalic acid. The reaction of PGwith TPA is quite slow as previously discussed. The resin produced on completion was also

relatively cloudy. Cloudy resins are a problem that we believe is due to the difficulties associatedwith dissolving a material that is “crystalline” or somewhat crystalline in a styrene solvent. Theterephthalate moiety is known to be particularly problematic in this regard and it takes a special

glycol to overcome this problem. Reaction with NPG, EG and DEG also produced problems withresin cloudiness. The product of NPG and TPA was so crystalline it would not dissolve at all in



styrene. Additionally the EG and DEG based UPR’s had relatively high moisture absorption. A

qualitative comparison of these systems can be seen in Figure 4.

Figure 3. Comparative reaction rate of PG andMPDiol with Isophthalic and Terephthalic Acid

Figure 4. Comparison of TPA-glycols reaction;reaction rate, polymer solubility in styrene, and

water absorption of UPR.

A problem that is observed with MPDiol is the high reactivity of this di-primary glycol in the last

hours (final stage) of reaction to form the finished unsaturated polyester polymer. As the resincook is concluding it is normal to heat the reaction to 200-210

oC for several hours, in order to

obtain a low acid number (typically about 20), and isomerize the maleate to fumarate (it is

desirable to obtain 90-95% or higher fumarate conversion). As these two reactions areproceeding, the polymer will increase in molecular weight, which of course will adversely effectthe finished resin viscosity. Therefore, producing a resin with the highest possible fumarate

content, while obtaining a suitably low acid number, and a polymer molecular weight thatgenerates a workable resin viscosity is tricky at best and normally leads to compromise inreaction conditions and polymer properties. The use of a di-primary glycol whether MPDiol or

DEG or NPG produces additional problems in the final stage of reaction, because the highreactivity of the glycol tends to favor molecular weight build (and therefore higher resin viscosity).Use of MPDiol (or one of the other di-primary glycols) to form a UPR, will typically produce a resin

with only 75 to 85% conversion of maleate to fumarate, with the expected loss in properties. Insome instances this loss in properties is acceptable, but it is never desirable. We havedeveloped several methods to reduce or eliminate these problems when using di-primary glycols,

so that high fumarate content can be achieved and the polymeric molecular weight can becontrolled. One of the easiest is adjusting the glycol stoichiometry so that a small amount of PGcan be introduced into the reaction. The lower reactivity of the PG slows down molecular weight

build in stage 2, and makes it possible to increase fumarate conversion. Conversion yields of maleate to fumarate of 94-98% are now routinely obtained, with acid numbers of approximately20 for the finished polymer, and molecular weights (Mn) = 2000-2500 with a polydispersity of

between 2 and 3, which provides a finished UPR with a very workable viscosity.



Conclusions: MPDiol (2-methyl-1,3-propanediol) is a glycol with unique characteristics that

make it very useful in synthesis of Ortho-, Iso-, and Terephthalate based Unsaturated Polyester Resins. The following advantages are observed when using MPDiol in making a UPR.

Increased Produ ction Rates: Because both hydroxyl groups of MPDiol are primary, it has an

inherently high reactivity. The high boiling point (30

o

C > PG) makes it possible to run reactionshotter, providing a potential eight fold increase in reaction (esterification) rates.

Improved Styrene Solubi l i ty : The unique structure of the MPDiol molecule produces a polyester polymer with reduced crystallinity and therefore increased styrene solubility. This reduction incrystallinity is particularly noticed in polyester systems that contain high proportions of

terephthalate segments, which are typically highly crystalline.Improved Corrosion Per formance: The use of MPDiol as the predominant glycol in a polyester resin, produces a UPR that demonstrates better corrosion resistance than similar polymers made

with PG, EG, or DEG. Large improvements are routinely observed in caustic corrosion resistanceImproved Mechanical Per formance: The UPR’s made with MPDiol demonstrate better.mechanical performance (strength and elongation to failure) than similar polymers made with PG.

Combined with the excellent corrosion resistance of these polymers, these UPR’s routinely retaina very high percentage of their initial properties following environmental exposure.

MPDiol ®

glycol is a registered trademark of the Lyondell Chemical Company.

Biographies

Lau Yang is a senior research fellow at Lyondell Chemical's Newtown Square, PA research

center. Dr. Yang received his Ph.D. in Chemistry from University of Chicago in 1974. His currentinterests are thermoset resins and composites.

Mac Puckett is a Market Development Manager working at Lyondell's corporate headquarters inHouston, TX. Dr. Puckett received his Ph.D. in Chemistry from Texas A&M University in 1982.

He has been working with thermoset resins, design, scale-up, application development andsupport, since he joined the industry in 1984.

Presented at SAMPE 2002

“Affordable Materials Technology – Platform to Global Value and Performance”May 12-16, 2002Long Beach, CA

2262-V1-0502

Unsaturated Polyester Resins

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